SOME SPECIAL PEOCESSES 255 



method is in practice somewhat laborious ; but probably 

 the results obtained, at least with suitable preparations, will 

 ultimately prove to be the best. 



The amount of contraction permissible with the sub-stage 

 iris-diaphragm must be determined by experiment. As in 

 visual work, this must not be carried to excess ; but possibly, 

 since contrast is only secured as the result of differences in light- 

 absorption, a rather narrower cone may be used than when 

 using ordinary objectives with white light. 



Another method of getting a sharp image is to do exactly 

 what was formerly done when using achromatic objectives not 

 corrected for photography. This is to determine the distance 

 between the actinic and the visual focus, and to make definite 

 allowance by means of the fine-adjustment before making the 

 exposure. In the case of the ultra-violet apparatus, an image 

 may be focussed with white light, and then a succession of 

 photographs taken with the ultra-violet apparatus, the fine- 

 adjustment of the microscope being moved a definite amount 

 each time. On developing, the image that gives the best and 

 sharpest result will be an indication of the amount that the 

 fine-adjustment must always be moved if the object is first 

 illuminated by white light and then the photograph taken 

 by means of the ultra-violet. A quartz micrometer-slide may 

 be obtained from Messrs. Zeiss, by means of which these 

 determinations may conveniently be made. 



As the process deals entirely with light of highly actinic 

 value, the particular plate used is not of great importance ; 

 except that, to ensure fineness of grain in the resulting 

 photograph, a slow one is preferable. No advantage is to 

 be gained by using orthochromatic plates ; in fact, any slow 

 process or lantern-plate will give a good result. It is also 

 not necessary to use a backed plate ; the light will not pass 

 through the glass and is in great part stopped by a gelatin 

 film, so that backing a plate becomes superfluous. 



As already stated, the preparations must be specially 

 prepared. Canada balsam and zylol are both opaque to ultra- 

 violet, and sections which have been passed through zylol 

 must have the latter completely removed. They may be 

 finally mounted in glycerin or in agar-salt solution. 



The method of photographing by ultra-violet light is 



